In an air-conditioning system performing ventilation, cooling, and heating, conditions that determine pressure loss of the air-conditioning system (hereinafter, “pressure loss conditions”) vary in accordance with the installation state, usage state, etc., of an air conditioner including an air blower. For example, the pressure loss conditions vary in accordance with differences in the shape and length of a duct connected to the air conditioner. In addition, clogging of filters fitted to a discharge port and a suction port of the air conditioner causes temporal changes in the pressure loss conditions. A static pressure necessary for obtaining a predetermined air flow rate also varies in accordance with the variation and temporal changes in the pressure loss conditions. For these reasons, even if a plurality of air conditioners of the same specifications are driven at the same torque or same rotation speed, air flow rates obtained from the respective air conditioners may vary, or there may be a case where even if one air conditioner is driven at a constant torque or constant rotation speed, the air flow rate of the air conditioner cannot be kept to a constant air flow rate. Therefore, in recent years, it is required to perform control to keep the air flow rate to a target air flow rate regardless of changes occurring in the pressure loss conditions and static pressure. (Hereinafter, such control is referred to as “constant air flow rate control”.)
As one example of a technique for realizing the constant air flow rate control, there is a technique using a pressure sensor. However, the technique using a pressure sensor has problems in terms of ease of installation and reliability under long-term use. Therefore, various techniques using no pressure sensor but utilizing characteristics inherent to an air blower (hereinafter, “blower characteristics”), the characteristics being necessary for the constant air flow rate control, have been studied. Such a technique requires, prior to starting an operation based on the constant air flow rate control, a measurement experiment to be conducted for measuring a relationship between physical quantities of the motor of the air blower (e.g., motor speed, motor torque, etc.) and the air flow rate of the air blower. Then, the constant air flow rate control is performed by utilizing blower characteristics obtained from the measurement experiment.
One of such known constant air flow rate control techniques is, for example, a technique disclosed by Patent Literature 1. In the conventional technique disclosed by Patent Literature 1, a designated speed of a motor driving an air blower is calculated from an equation (1) below. [Math. 1]S*=S+K×(S−Sa)  (1)
In the equation (1), S* is a designated motor speed; S is a motor speed; Sa is a target motor speed; and K is a gain. The target motor speed Sa is calculated from an equation (2) below.
                    [                  Math          .                                          ⁢          2                ]                                                            Sa        =                              ∑                          n              =              0                        j                    ⁢                      (                                          ∑                                  m                  =                  0                                i                            ⁢                              (                                                      k                    nm                                    ⁢                                      T                    n                                    ⁢                                      Q                                          *                      m                                                                      )                                      )                                              (        2        )            
In the equation (2), T is a motor torque; Q* is a target air flow rate; knm (n=0, 1, 2, . . . , j and m=0, 1, 2, . . . , i) is a constant; and i and j are finite values. The equation (2) represents the following relationship: in a case where the motor torque is T, a motor speed necessary for the air flow rate to become the target air flow rate Q* is Sa.
Hereinafter, constant air flow rate control using a motor control device disclosed in Patent Literature 1 is described with reference to FIG. 6. FIG. 6 illustrates the conventional constant air flow rate control technique using the motor control device.
In FIG. 6, a curve 601 represents torque-speed characteristics that allow the air flow rate to become the target air flow rate Q* (hereinafter, the curve 601 is referred to as a “constant air flow rate curve”); and a curve 602 represents motor torque-speed characteristics specific to pressure loss conditions under which the motor is installed (hereinafter, the curve 602 is referred to as a “pressure loss curve”). Assume here that the operating point of the motor is currently a point A1 on the pressure loss curve 602. At the time, the target motor speed Sa calculated by using the equation (2) is a motor speed Sa1 at a point B1 on the constant air flow rate curve 601, at which a motor torque T1 is obtained, which is the same torque as that obtained at the current operating point A1. Then, based on the equation (1), the motor speed Sa1 is subtracted from the motor speed S1 at the current operating point A1, and the resultant difference (represented by ΔS in FIG. 6) is multiplied by the gain K and added to the motor speed S1. The value resulting from the calculation is outputted as the designated motor speed S* (not shown). Then, the motor speed is controlled in accordance with the designated speed S*, and thereby the operating point moves.
After the operating point has moved, the designated speed S* is newly calculated by using the equation (2) and the equation (1) in the same manner. As a result of repeating the calculations using these equations, the operating point moves to A2 and A3 as shown in FIG. 6, and eventually reaches an operating point at which the difference between the motor speed S and the target motor speed Sa is zero, i.e., reaches an intersection point E of the pressure loss curve 602 and the constant air flow rate curve 601.
The motor control device disclosed in Patent Literature 1 calculates a necessary correction value to the motor speed by using a motor torque and a target air flow rate in the above-described manner, and outputs a designated speed based on the correction value, thereby making it possible to perform constant air flow rate control that is not affected by changes in the pressure loss conditions and static pressure.